Referencing this part of the API:

https://lua-api.factorio.com/latest/types/InventoryWithCustomStackSizeSpecification.html

I was able to get merging chests working with steel chests on normal mod settings using said stack size multipliers to increase the "size" of the inventory rather than increasing slot count. I will be looking into it further.

If I understand it correctly I would be able to specify that one inventory slot can contain for example 10 stacks worth of items. I wonder how this will work with unstackable items like armor. But could be interesting to avoid scrollbars in merged chests.

But since I have mod setting for setting inventory type (one possibility being "with_filters_and_bar") I would still have to have fallback to old logic...at which point I wonder if it is worth it